1. Field of the Invention
The present invention relates to new aqueous coating compositions for coating metal substrates, such as cans, which satisfy in particular the requirements from the food sector and to a process for preparing them.
2. Description of the Prior Art
For producing containers, such as cans, for use as packaging material, particularly for the packaging of foods, including beverages, sheets of tinplate, chromated steel and aluminium in panel or strip form typically are coated. This coating prevents contact between contents and metal, so that corrosion of the metal or any effect of the metal on product quality is ruled out. Nor of course must the lacquer film itself—as a result of lacquer constituents which have leached out, for example—result in any influencing or adverse effect on the contents. Coating materials and coatings for cans are therefore subject to very exacting requirements in respect of flexibility, solvent resistance and chemical resistance, and toxic constituents.
Typically the so-called gold lacquers based on high molecular mass epoxy resins and phenolic resins of the resole type, which are partly etherified or otherwise plasticized, are used as interior protective lacquers for metal packaging. It is possible in this case to apply two or more lacquer films, including films based on different resins. This is necessary, for example, if the requirement is for particularly high-grade corrosion protection in conjunction with a critical metal deformation and/or a particularly long filled-can storage time.
Likewise employed are epoxy-amine resin lacquers, although such lacquers are more sensitive towards adverse metal surfaces and also have a lower resistance to acidic contents.
Lacquers based on vinyl chloride copolymers, for which points of note include in particular absence of flavour and a low pore count, have uses which include the interior protection of beverage cans. PVC organosols as well are employed, particularly for tear-open lids made of aluminium and from tinplate, noteworthy features here being the high achievable solids content and the good film elasticity, even when the layer is applied in a relatively thick film.
The prior art, described in “Internal Lacquers of metallic food containers”, K. R: Kumar, in Popular Plastics & Packaging, August 2001, pages 66-70, includes the use of coating compositions based on epoxy resins and phenolic resin hardeners.
WO 88/01287 and U.S. Pat. No. 4,018,848 also disclose coating compositions based on epoxy resins and phenolic resin hardeners for coating metal strips for producing metal cans for foods. These coatings are distinguished by a particularly good resistance to aggressive products, good mechanical properties, such as high elasticity, and very good metal adhesion. Nevertheless virtually all of these coating compositions comprise organic solvents, examples being aromatics mixtures such as Solvesso® 100, 150 or 200. The epoxy resins used virtually all include 2,2′-bis(4-hydroxyphenyl)propane bis(2,3-epoxypropyl) ether (or homologues thereof), which is also known as bisphenol A diglycidyl ether or “BADGE” (Bisphenol A Di-Glycidyl Ether).
Bisphenol A (BPA), the precursor of “BADGE”, is known on account of its effect on the endocrine system. It is also known that “BADGE” is mutagenic in in vitro tests and that small amounts of this substance may enter the packaged foodstuffs. Additionally “BADGE” is suspected of being carcinogenic. It is therefore advantageous to use BADGE-free coating compositions for the interior coating of cans with foodstuff contact.
“BADGE”-free coatings are described for example in DE-A 199 12 794 and EP-A 1 277 814. These coatings, however, are based on solvent-borne coating compositions with a considerable solvent fraction (up to 90% by weight), which on emission grounds are nowadays unwanted. Furthermore, residues of solvent in the coatings may migrate and thus adversely affect the contents.
Aqueous coating compositions for can coating applications are known, but they contain either blocked polyisocyanate crosslinkers or binders synthesized from components which do not comply with § 175300 of the USA Code of Federal Regulations No. 21, FDA (Food and Drug Administration).
In the course of the baking operation, blocked polyisocyanates give off the corresponding blocking agent, e.g. butanone oxime or ε-caprolactam. These elimination products may, however, remain partly in the lacquer film and hence migrate into the subsequent contents. Such systems have therefore hitherto been unsuitable for use in the food sector.
It was an object of the present invention, therefore, to provide “BADGE”-free aqueous coating compositions which are FDA-compliant in accordance with Paragraph 175300, USA Code of Federal Regulations No. 21, contain not more than 20% by weight of organic solvents and possess as a crosslinked film very good mechanical properties, such as high elasticity; a high level of resistance to solvents, chemicals and fatty and oily substances, and which at the same time exhibit high resistance under thermal stress.
FDA-compliant in this context means that the components present in the binder compositions or used in their preparation are exclusively those listed in § 175300 in the USA Code of Federal Regulations No. 21 (FDA).